Higher level walls should have force fields

I think it would be nice to see walls play a larger part in strategies of aerial attacks.

Starting at level 7 walls any trajectory going over the wall should have decreased damage and any enemy that is jumping or flying over the wall should have a slight and temporary speed reduction.

so an archer shooting over a level 7 wall would have -1% damage to its target. A dragon flying over a level 7 wall should decrease in speed by 1% for 1 tile.

then level 8 walls could be 2% decrease, level 9 3% decrease etc.